March 2015 made climate history, and not in a good way

For the first time on record in human history, the monthly global average concentration of carbon dioxide — the most important global warming pollutant that takes centuries to be removed from the atmosphere — surpassed 400 parts per million in March of this year, according to government data.

The world first briefly bumped up against the 400 parts per million barrier in May 2013, although some observation sites in the Arctic hit that mark a year prior. And last April, the Mauna Loa Observatory in Hawaii reported monthly levels of carbon dioxide above 400 ppm. However, this was not shared by all global sites.

This means that March 2015 was the first time that the Earth as a whole surpassed the 400 ppm mark for the entire 31-day period, according to the National Oceanic and Atmospheric Administration (NOAA).

Carbon dioxide and other greenhouse gases, such as methane, warm the planet by absorbing and redirecting outgoing solar radiation that would otherwise escape back into space.

NOAA bases Wednesday's announcement on data from 40 sites in both hemispheres. These sites, which are situated on ships and at isolated islands, are collected on cargo ship decks, remote islands and at other locations far from local pollution sources, such as factories and power plants. The most famous of these sites is at the summit of Mauna Loa in Hawaii, where the carbon dioxide record began in the 1950s.

According to NOAA, it takes some time after each month's end to compute the global average because samples are shipped from locations for analysis at NOAA's Earth System Research Laboratory in Boulder, Colorado.

Ed Dlugokencky, the NOAA scientist who manages the global network, expects the global average to remain above 400 ppm through May, which is the typical annual peak due to natural cycles that occur on top of the long-term rising trend of greenhouse gases. Carbon dioxide levels drop back down as plants in the northern hemisphere begin to bloom, using carbon dioxide for photosynthesis in late spring and summer, which temporarily lowers the amount of this gas in the air.

When the Mauna Loa data collection first began in 1958, CO2 levels were at just 313 ppm, indicating an extraordinarily steep rise — recently about 2 ppm per year — that has been attributed to human activities, such as the burning of fossil fuels such as coal, oil and natural gas. The weekly Mauna Loa average CO2 level for the week beginning on April 26 was about 404 ppm, according to NOAA data.

Carbon dioxide, or CO2, is the most important long-lived greenhouse gas responsible for manmade global warming, and once emitted, a single molecule of carbon dioxide can remain aloft for hundreds of years, which means that the effects of today's industrial activities will be felt for the next several centuries, if not thousands of years.

Studies have shown that CO2 levels have not been this high in at least 800,000 years, and possibly as long as 15 million years.

"It was only a matter of time that we would average 400 parts per million globally," said Pieter Tans, lead scientist of NOAA's Global Greenhouse Gas Reference Network, in a NOAA press release.

"This marks the fact that humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120 parts per million since pre-industrial times," Tans says. "Half of that rise has occurred since 1980." NOAA data show that the average growth rate of carbon dioxide concentration in the atmosphere from 2012 to 2014 was 2.25 ppm per year, the highest ever recorded over three consecutive years.

Long-term record of carbon dioxide concentrations during the past 800,000 years.

Image: Scripps Institution of Oceanography/Mashable

This increase was despite findings by other organizations, such as the International Energy Agency, which reported on March 13 that the growth of global emissions from fossil fuel burning stalled in 2014, remaining at the same levels as 2013. This may be a sign that emissions trends are separating, or decoupling, from economic growth, which is a longstanding policy goal for many world leaders.

However, to reverse the rising trend of carbon dioxide levels, steep emissions cuts will need to be made, all the way to negative emissions by the end of the century.

James Butler, director of NOAA's Global Monitoring Division, added that it would be difficult to reverse the increases of greenhouse gases which are driving increased atmospheric temperatures.

"Elimination of about 80% of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere, but concentrations of carbon dioxide would not start decreasing until even further reductions are made and then it would only do so slowly," Butler said in a press release.

In another year or two, it is likely that the 400 ppm threshold will be exceeded year-round, as the world races toward a future with atmospheric concentrations of CO2 approaching 450 ppm or more by the end of the current century, even if countries begin undertaking sharp emissions reductions in the near future. Some climate scientists consider about 350 ppm as a "safe level" of CO2 to avoid the worst impacts of global warming, while others argue that it may be slightly higher, or even lower, than that.

There is wide scientific agreement that continuing down the business as usual path of emissions would be disastrous as polar ice caps melt, flooding coastal megacities around the world, and altering weather patterns, with deadly consequences.

The new data may serve to boost efforts to reach a new and effective global warming agreement at a crucial round of global climate talks in Paris at the end of the year.

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